1. Field of the Invention
The present invention relates to a power saving control system for a computer system. More specifically, the invention relates to a power saving control system particularly applicable for a battery-operated portable personal computer. Further particularly, the invention relates to a power saving control system which can selectively initiate power saving mode operation depending upon an address access repetition cycle.
2. Description of the Related Art
As disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2-178818, for example, power saving control is well-known in the art to interrupt power supply to those sections of a computer system which are not executing any substantial task for reducing total power consumption of the computer system. Such technology has already been applied to field products. In the field of battery-operated personal computers including a lap-top computer, much study has been given in the power saving technology of this type for the sake of maximizing the system uptime with a smaller and lighter battery.
Conventionally, two types of functions for placing the personal computer into a stand-by state, i.e., a so-called rest mode function and a sleep mode function are provided in certain types of personal computer systems. The rest mode function is performed for automatically switching an operational clock frequency of the computer system from 16 Mhz in the normal operational mode to 1 Mhz when a CPU is held inoperative state for a predetermined period of time. If a further predetermined period elapses while the computer system is held in the rest mode state, the sleep mode is automatically initiated to shut down the power supply. In either mode of operation of the computer system, the normal mode operation can be resumed by operating an arbitrary key. In many cases, the predetermined period of time for initiating the stand-by mode can be arbitrarily set by the user through manual setting operation.
As can be appreciated, conventionally, the computer system is placed into the power saving state (i.e. the above-mentioned stand-by mode) in terms that the "the CPU is held in a substantially inoperative state for a predetermined period of time". In practice, the operational state of the computer system is changed over from the normal state into the power saving state in absence of any external factor for initiating any substantial task of the CPU, such as input through entry from a keyboard or from a communication supervisory system or so forth, for the predetermined period of time.
In general, the CPU is regarded as being in a substantial rest state when the above-mentioned factor to cause a substantial task is absent for the predetermined period of time. With the conventional technology, in which the foregoing substantial rest state of the CPU is detected to change over the operational mode into the power saving mode, however, such procedure may not achieve sufficient power saving because it requires to set the predetermined period of time more than several tens of seconds for a typical personal computer.
For instance, it is assumed that a word processing application is active in the personal computer. In this case, every entry through the keyboard serves as the factor for causing initiation of the task in the CPU. In response to each input signal, the CPU performs various tasks including very simple tasks, such as displaying a character on a display screen, slightly time-consuming tasks, such as conversion of Kana entry into Kanji character or moving of a document and time-consuming tasks, such as sorting of files or so forth. When an operator is drafting a document, he tends to have relatively long intervals between key entries for considering word, phrase or so forth. In such case, the possible interval between key entries over several tens of msec. to several sec. frequently occurs.
In this circumstance, if the predetermined period of time in the conventional power saving method is set in a range of 1 sec. to several sec., the power saving mode operation can be initiated during time-consuming tasks to cause failure of the on-going task. For avoiding such possibility, it becomes necessary to set the predetermined period in a range of several tens of sec. or several min. In the latter case, the power saving function is held inoperative during frequently occurring short intervals of key entry, in which the CPU is held substantially resting, so that satisfactory power saving effect cannot be achieved.
As a solution for this, it is possible to make the CPU per se declare a substantial rest state thereof to an external circuit (power saving circuit) every time the substantial rest state occurs. However, in such case, the function has to be included in all software to be executed by the CPU. It is difficult or even impossible to install such function in software which has already been completed.
On the other hand, if the computer system executes a single piece of software, it is possible to precisely detect the substantial rest state of the CPU by preliminarily analyzing the content of the software and detecting the state, in which a small loop is repeated, accessing only a specific address group. However, this approach probably is not practical when the computer system is active in a plurality of software.